1. Field of the Invention
The present invention relates to a method and apparatus for regulating the voltage of single phase power feed lines connected to adjacent electrically isolated a.c. busbar sections in an electric railway power distribution system.
1. Brief Description of the Prior Art
In conventional electric railway traction systems, trains run along railway tracks which are at electrical ground, while single phase power is distributed variable to electric trains (i.e. variable railway loads) by an a.c. busbar which, in a catenary type system, is realized as wiring running above the railway tracks. Typically, adjacent pairs of a.c. busbar sections are electrically isolated from each other by an open circuit or dead section, referred to as a midpoint or junction along the a.c. busbar. As illustrated in FIG. 1, alternate pairs of adjacent a.c. busbar sections 1 and 2 separated by open circuit midpoint section 3 are fed with a first phase pair 5 provided to a power supply substation 4 from a three phase power supply system (not shown). As shown, first phase pair 5 is operably connected to a.c. busbar sections 1 and 2 using a transformer 6 and in-line circuit breakers 7. Similarly, adjacent a.c. busbar sections 8 and 13 separated by open circuit midpoint section 3' are fed with a second phase pair provided to power supply substation 4' from the three phase power supply system. Also shown, second phase pair 5' is operably connected to a.c. busbar section 8 and 13 using transformer 6' and in-line circuit breaker 7'. Due to the configuration of the power distribution system, power supply substations 4 and 4' provide different sources of single phase a.c. power to a.c. busbar sections 2 and 8, disposed on opposite sides of open circuit midpoint section 9 along the a.c. busbar.
When a pantograph 1O of electric train 11 slides onto midpoint section 9, single phase power supplied to the electric train from power supply substation 4, is automatically disconnected. While the pantograph slides along midpoint section 9, there is no contribution of power from a.c. busbar sections 2 or 8. Then, when the pantograph slides from midpoint section 9 onto a.c. busbar section 8, single phase power from power supply substation 4' is automatically provided to the pantograph. During this sliding "make-before-break" transfer of the pantograph along the a.c. busbar, a.c. busbar sections 2 and 8 are never electrically shorted. If the electrically isolated midpoint section 9 did not exist, undesirable reactive power flows would occur between a.c. busbar sections 2 and 8, as the a.c. power along these different a.c. busbar sections is out of phase due to loading conditions along these sections and the phase-relationship of the first and second phase pairs provided to power substations 4 and 4'. Typically, this has a significant influence on the voltage regulation of the electric railway system.
There are, of course, other factors which influence voltage regulation within electric railway power distribution systems of the type described above. Such factors include, for example, constantly changing grades along which trains move over the railway; operating conditions which necessitate that trains decelerate for speed restrictions, crossings with other trains, slowing and stopping for railroad signals; and required acceleration after stopping to pick up passengers, or to avoid encountered conditions along the railway track.
In order to improve the voltage regulation and load factor of prior art electric railway power distribution systems, a number of proposals have been set forth and implemented. These prior art proposals are discussed at length in Applicant's paper entitled "Methods Of Improving The Voltage Regulation Of 25Kv 50Hz Electric Railways," published in the Fourth International Heavy Haul Railway Conference, September 1989. Such prior art techniques include, for example, (i) reducing the distance between high voltage power supply substations, (ii) providing switching stations between these power supply substations, (iii) providing series or shunt connected compacitors to reduce active load along the power distribution system; and (iv) providing transformer-type regulations in order to provide voltage regulation. As discussed in Applicant's paper, each of these approaches suffers from a number of shortcomings and drawbacks, including the high cost of implementation or other adverse effects due to high consumption of electrical power.
One approach used by the French National Railway System involves installing equipment at power switching substation 12 in order to galvanically connect a.c. busbar sections 2 and 8 at midpoint section 9, as these a.c. busbar sections are fed single phase power from synchronized power supply substations 4 and 4'. The purpose of this switching scheme is to connect these a.c. busbar sections in an electrical parallel configuration. In France, essentially constant loading conditions exist along the power distribution system, and thus permit single phase a.c. feed supplies from adjacent power substations to remain sufficiently in phase. As a result, fast protection can be installed and voltage regulation along the electric railway power distribution system can be halved by the galvanically connected adjacent a.c. busbar sections at open circuit midpoint sections along the a.c. busbar. However, in Australia and other places where variable loading conditions are expected along the power distribution system, this paralleling technique is generally unacceptable.
Thus, there is a great need in the electric railway art for a way in which to interconnect unsynchronized and synchronized but out of single phase power feed lines supplying power to electrically isolated a.c. busbar sections of an electric railway power distribution system, while achieving voltage regulation along the system and avoiding the shortcomings and drawbacks of prior art methodologies.